1. Field of the Invention
The present invention relates to an optical pickup apparatus.
2. Description of the Related Art
As an optical pickup apparatus compatible with a plurality of types of optical discs different in track pitch, there is known an optical pickup apparatus employing tracking control by an inline differential push-pull method. By the inline push-pull method, laser light emitted from a laser light source is diffracted by a diffraction grating in which regions having periodic structures different in phase from each other are joined, to generate 0th order light and ±1st order diffracted lights. The 0th order light and ±1st order diffracted lights are applied to a recording layer of the optical disc, and reflected lights thereof are received, to generate a main push-pull signal and a sub push-pull signal. Then, by generating a differential push-pull signal to obtain a tracking error signal from the main push-pull signal and the sub push-pull signal, an offset component accompanying displacement of an objective lens or inclination of the optical disc can be effectively reduced. It is known that, in the diffraction grating, by providing a central region having a periodic structure further different in phase between the regions including the periodic structures different in phase from each other, a visual-field characteristic can be improved which indicates a deterioration rate of the differential push-pull signal when the objective lens is displaced in a tracking direction (radial direction of the optical disc)(Japanese Patent Laid-Open Publication No. 2004-145915, for example). Also, there is known an optical pickup apparatus including a laser light source for emitting laser light having a wavelength corresponding to each of the optical discs in order to be compatible with both CD (Compact Disc) and DVD (Digital Versatile Disc)(Japanese Patent Laid-Open Publication No. 2007-220175, for example.)
The ±1st order diffracted lights diffracted by the diffraction grating further becomes reflected lights of the 0th order light and the ±1st order diffracted lights, respectively, by a diffraction function of a land and a groove formed in the recording layer of the optical disc. Then, by the 0th order light and the ±1st order diffracted lights, which are the reflected lights, sub beam spots are formed on a photodetector for generating the sub push-pull signal. In these sub beam spots, lightness of a region where the 0th order light and the +1st order diffracted light overlap and lightness of a region where 0th order light and the −1st order diffracted light overlap change according to a tracking position of the optical pickup apparatus. Then, the sub push-pull signal is generated based on a difference between a detected value on the +1st order side and a detected value on the −1st order side on the photodetector.
In a case where the central region is provided on the diffraction grating as shown in Japanese Patent Laid-Open Publication No. 2004-145915, as shown in FIG. 8, a region with different lightness having a width D corresponding to the central region of the diffraction grating is generated at the center of the 0th order light forming the sub beam spot. Therefore, if the width D in the 0th order light is too great, a signal level of the sub push-pull signal becomes small, and a SN ratio (Signal to Noise Ratio) of the sub push-pull signal may be lowered. Particularly in the case of being compatible with both CD and DVD, it is required to satisfy a required level on a visual-field characteristic as well as suppress lowering of the SN ratio of the sub push-pull signal, for both the optical discs.